Depends on which Cessnas we are talking about. O-2 Skymaster aircraft were often used as FACs during VietNam, and these could carry marking flares, 2.75" Hydra rockets (maybe called Mighty Mouse back in those days), and even small bombs and 7.62mm minigun pods. But I doubt that the current liaison and basic flight training aircraft still in service (although I'm pretty sure that the C-12 is a Beechcraft product) have any provision whatsoever for any external hook-ups, even small fuel tanks, let alone any useful internal space (wings or nose) to successfully install any gun (even MG caliber.) Raytheon's "new" T-6 turboprop trainer has been weapons-qualified with light armament for pilot training/familiarization. Matter of fact, Sidewinder-equipped models were operated by Greece during the Summer 2004 Olympics, as it was proposed they would appear less threatening than combat jets. In various forms, most modern turboprop trainers can sacrifice a wing fuel tank each side to internally mount single guns, typically 12.7mm MGs Anything else would need to be podded (GIAT markets a 20mm aircraft pod gun based off their M621 used on APCs.) But as far as other utility/liaison aircraft, certainly some ingenious mechanic could jury-rig an installation, but it would be little more than field-expedience in a time when sh*t really hit the fan and absolutely nothing else is available or left. And these light utility aircraft would probably have difficulty absorbing the recoil of a 1700-shots-per-minute BK27 anyway. But certainly they could handle MG-caliber weapons.

I am familiar with mounting guns on helicopters. The first consideration with an existing airframe is where do yo put it. Most military aircraft have hardpoints connected directly to the airframe. The carrying capacity and structural integrity of these mounts limits your choices (recoil,. weight X G-load, etc.). Few light aircraft can handle .50 cal recoil. Next comes where do you put ammo. This is usually the dealbreaker in a light aircraft. MG's use 400-600 RPM; RBC's use 3000-4000 RPM. You need at least a minute of firing for RPC's and 2 for MG's. .50 cal gun weighs 175 lbs, fixed mounts 75 lbs minimum, each round 1/2 pound so 600 rounds = 300 lbs. Total for fixed forward install at least 550 lbs. This weight comes straight off the useful load. Max Gross - Empty weight - crew - gun - required mission fuel = useful load

I am familiar with mounting guns on helicopters. The first consideration with an existing airframe is where do yo put it. Most military aircraft have hardpoints connected directly to the airframe. The carrying capacity and structural integrity of these mounts limits your choices (recoil,. weight X G-load, etc.). Few light aircraft can handle .50 cal recoil. Next comes where do you put ammo. This is usually the dealbreaker in a light aircraft. MG's use 400-600 RPM; RBC's use 3000-4000 RPM. You need at least a minute of firing for RPC's and 2 for MG's. .50 cal gun weighs 175 lbs, fixed mounts 75 lbs minimum, each round 1/2 pound so 600 rounds = 300 lbs. Total for fixed forward install at least 550 lbs. This weight comes straight off the useful load. Max Gross - Empty weight - crew - gun - required mission fuel = useful load